This invention relates to an improved intraocular implant (pseudophakos) which may be permanently placed in the anterior or posterior chamber of a human eye for the correction of aphakia and re-establishment of binocularity in aphakia.
Intraocular implants have a lens and a haptic for fixation of the lens, by a surgeon, in the anterior or posterior chamber of the human eye. The intraocular lenses can be used to replace the natural and heretofore have been made primarily from polymethylmethacrylate (PMMA). The haptic may be of a variety of shapes depending, to some extent, on whether the lens is to be implanted in the anterior or posterior chamber of the eye. For example, in an article entitled "The Iridocapsular (Two-loop) Lens and Iris-Clip (Four-loop) Lens in Pseudophakia", September-October, 1973, edition of Transactions of the American Academy of Ophthalmology and Otolaryngology, there is described a surgical procedure for implanting both two-loop and four-loop intraocular lens on the iris of a human eye. These lenses were made from PMMA.
U.S. Pat. No. 4,198,714 discloses a method for implantation of an intraocular lens, preferably made from PMMA, into the posterior chamber of the hauman eye within the capsular membrane thereof. It is disclosed that the lens was implanted by removing a portion of the capsular membrane so that the intraocular lens could be inserted behind the iris. A first loop attached to the lens was placed in a pocket formed by the remaining portion of the capsular membrane. The iris is then pulled around the second supporting loop so that the entire lens together with its loops is placed into the posterior chamber of the human eye. It is disclosed that the anterior side of this pocket and the posterior side of the pocket eventually scar together, thereby securing the lens within the posterior chamber. Once the posterior and anterior side of the pocket of the capsular membrane have scarred together, the intraocular lens is said to be firmly, permanently and securely fixed to the capsular membrane and the iris is free to function normally. It is also taught that the use of an integral molded member eliminates edge reflections which occur in lenses implanted in the anterior chamber and internal reflections which are caused by posts for the supporting loops in other lenses implanted in the posterior chamber.
In addition to the loop attachment means described in U.S. Pat. No. 4,198,714, intraocular lenses can have other haptics such as Hessburg or J-Loop designs as well as other diverse haptic means.
Most haptics which flex, such as the haptic loops, are made of polypropylene. Polypropylene is not as brittle and has greater mechanical strength and toughness than PMMA. Nevertheless PMMA is the present plastic of choice as the lens material because it has good optical properties and has demonstrated excellent biocompatibility in the eye. However, PMMA has several disadvantages. For example, PMMA is a relatively brittle, glassy polymer with a low glass transition temperature (Tg) of about 90.degree. C., a low thermal stability and relatively poor mechanical strength. In addition, PMMA also transmits a substantial portion of ultraviolet light which may be harmful to the aphakic eye. In an attempt to overcome this problem prior art workers have added UV absorbers.
Because of PMMA's low Tg and low thermal stability, intraocular implants made of PMMA cannot be sterilized by autoclaving with steam because of the high temperatures employed (about 120.degree. C.-125.degree. C.). At such temperatures PMMA becomes soft and distorted. Moreover, PMMA lenses are not sterilized using high energy radiation, e.g. Gamma sterilization at from 2 to 5 megarads, because of uncertainties regarding PMMA degradation and resulting problems of biocompatibility. Accordingly, PMMA intraocular implants are sterilized by techniques such as ethylene-oxide or other chemical or gas sterilization methods.
The use of ethylene oxide and the like for sterilization has serious disadvantages, and the Federal Drug Administration has expressed its concern by establishing maximum gas retention levels and it is known that excessive amounts may result in serious chemical burns on skin and mucous membrane irritation. Furthermore gas sterilization, for plastic which cannot withstand steam autoclave sterilization such as PMMA, takes a longer time which is not always available, or if available, can be very costly, as is the case in surgery where both labor and facilities are very costly. Ethylene oxide and its derivatives are low boiling ethers which present an added danger and therefore require addition precautions not required with steam autoclave sterilization. Ethylene oxide and other "cold" sterilization procedures are more expensive than steam autoclave sterilization.
Accordingly it can be appreciated that sterilization procedures for implantation of intraocular lenses need to be very short so that a minimum amount of time is required. Chemical and gas sterilization methods are generally reserved for those incidences where steam and radiation sterilization cannot be employed.
Although PMMA has heretofore been the plastic generally used for lenses it was not generally used as the haptic. However, recently there has been emphasis on a one piece or integral intraocular implant and since PMMA was the material generally used to make the lens, PMMA has been suggested and used as the haptic in integral implants. As a haptic material PMMA is much more disadvantageous than as a lens material. PMMA haptics cannot, of course, be sterilized using steam or radiation. In addition, because of PMMA's poor mechanical strength and lack of ductility, (i.e. PMMA has a notched Izod impact strength of one-eighth inch thickness of about 0.3-0.4 ft. lbs./in.) it is possible that some haptics made of PMMA will not withstand the flexing in the eye without ultimately breaking. This would be a very serious problem. Even as a lens material, PMMA's lack of mechanical strength requires thicker lenses than if a stronger plastic were used. In addition stronger and tougher plastics would allow one skilled in the art to make more versatile lens designs.
Accordingly, there is a need for an intraocular implant material which is strong, ductile, easily machined or molded into thin sections, having a specific gravity of less than 2, preferably less than 1.7, and especially preferably about 1.4 or lower, which is chemically inert and stable and capable of withstanding autoclave steam sterilization without softening or permanent distortion or high energy radiation sterilization without causing an adverse change in properties.
Furthermore, there is a need to improve the effectiveness and speed with which an intraocular implant having the desired specific gravity and mechanical properties can be sterilized.
In addition, there is also a need, under certain circumstances, for an intraocular implant to be stable and withstand the energy when a laser beam is passed therethrough. For example, after removal of cataracts and the implantation of a posterior intraocular lens, in a large number of cased the posterior capsule becomes opaque. This obscures the vision and therefore requires removal of a small portion of the capsule which is located horizontally to the intraocular lens. This removal of a portion of the posterior capsule is preferably done by passing a laser beam through the intraocular lens and burning a hole in the capsule. Thus the lens must be stable to laser energy and therefore it is preferable if the lens has greater stability and strength than is possible using PMMA.
It is therefore, an object of this invention to provide an intraocular lens and/or haptic having superior mechanical thermal and sterilization properties to that of PMMA intraocular implants.